Many health groups, including the National Osteoporosis Foundation, recommend bone density testing every year or every two years for people taking bisphosphonates like Fosamax, Actonel, Reclast or Boniva, or other types of bone-strengthening drugs.

But the new study shows the tests to be of little value in determining how well a patient is responding to bisphosphonate treatment.

The vast majority of patients in the trial showed improvement during the first few years of treatment and there was little variability in response from patient to patient.

"Measuring bone mineral density is important for diagnosing osteoporosis and determining who should be on treatment, but for people who are in their first few years of treatment it does not appear to be useful," study co-author Les Irwig of the University of Sydney tells WebMD.

Accuracy of Bone Mineral Density Testing

In an effort to access the value of routine bone density monitoring during treatment, Irwig and colleagues analyzed data from a study that included more than 6,000 postmenopausal women treated with Fosamax or placebo for three years.

Bone mineral density testing was performed at the start of the study and then each year throughout.

After three years of treatment, 97.5% of the women treated with Fosamax showed at least modest increases in hip bone mineral density and the treatment effect did not vary significantly between patients.

But there was quite a lot of variability in the measurements from year to year among individuals, Irwig says, suggesting that the test is not very accurate and could be misleading.

The study appears in the latest issue of BMJ Online First.

"A test may show a decline in bone density when this isn't the case," he says. "This would give the patient the impression that the drug is not working when it is."

Even if the test were completely accurate, bone mineral density testing is not a particularly good measure of fracture risk, says Juliet Compston, MD, University of Oxford professor of bone medicine.